Apparatus and method for eliminating artifacts in active pixel sensor (APS) imagers

ABSTRACT

An active pixel sensor (APS) that includes circuitry to eliminate artifacts in digital images. The APS includes a comparator for comparing a signal level from a pixel to an adjusted saturation voltage to determine if the pixel is saturated. If the pixel is saturated, the signal output from the pixel is replaced with an analog voltage having a maximum value corresponding to a brightest pixel in the image.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/123,488 entitled AN ANALOG SOLUTION FOR OVERSATURATED PIXEL PROBLEMfiled on Mar. 9, 1999.

BACKGROUND

Active pixel sensor (APS) devices are described in U.S. Pat. No.5,471,515. These devices include an array of pixel units that convertlight energy into electrical signals. Each pixel includes aphotodetector and one or more active transistors.

Unlike charge coupled devices (CCD), APS imaging devices are compatiblewith complementary metal oxide semiconductor (CMOS) technology. Thisfeature enables integration of imaging electronics and digital imagesignal processing electronics on the same chip in APS devices. Suchintegration can provide advantages in design and performance.

Conventional CMOS imaging devices can be prone to producing artifacts inthe resulting image. Artifacts are objects in the image that appearblack when in fact they should be the brightest regions of the image.

Artifacts can occur in oversaturated pixels in the APS. Oversaturatedpixels are pixels that are exposed to more light energy than the pixel'sphotodetector can absorb during exposure time.

This is mainly a problem in very high contrast images. The extremebrightness of the sun will produce artifacts in virtually all CMOSimagers, even at very short integration times. This is a disconcertingeffect in outdoor imaging situations, in which the sun is oftenaccidentally included in the field of view.

FIG. 1 illustrates such a situation in a digital image 10 produced usinga conventional APS CMOS imager. The presence of the sun 12 in the imageproduces an artifact 14 in which the center of the sun appears black.

SUMMARY

An active pixel sensor according to an embodiment includes at least onepixel having a photodetector that outputs an output level indicative ofincoming light. A storage element stores said output level during aphotodetector readout operation. A comparison element compares theoutput level to an adjusted saturation voltage. An output selectorelectrically connected to the comparison element selectively outputs anadjusted maximum voltage during a pixel readout operation in response tothe comparison element determining that the adjusted saturation voltageexceeds the output level.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a digital image including an artifact produced in the centerof the sun caused by oversaturated pixels.

FIG. 2 is a schematic diagram according to one embodiment.

FIG. 3 is a chart showing the response of voltage levels to incidentlight levels according to the embodiment of FIG. 2.

FIG. 4 is a schematic diagram of another embodiment including digitalprocessing circuitry.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Oversaturated pixels in an active pixel sensor (APS) can produceartifacts in the resulting images in high contrast situations. Artifactsare objects in the image that appear black, but in fact should be amongthe brightest objects in the image.

According to one embodiment, the value read out from an oversaturatedpixel is replaced with a predetermined maximum value corresponding to amaximum brightness for the pixels in the image. This eliminates anyartifacts in the resulting image. The pixels in the APS array thatreceive the most light appear brightest in the images that are produced.

FIG. 2 illustrates an APS 20 according to an embodiment that includes anarray of independently addressable pixels arranged in n rows and mcolumns. An independently addressable pixel 22 in a row 24 includes avoltage source set at V.aa, typically set at about 3.3 V to 5.0 V, aphotodiode 26, a reset transistor 28, a source-follower outputtransistor 30, and a row select transistor 32. Pixels in each row areconnected to a column 34. The bottom of each column of pixels has a loadtransistor 36 at a voltage V.ln, a signal branch 38 to sample and storethe signal level for readout, a reset branch 40 to sample and store thereset level for readout, and a comparator branch 42 to sample and storethe signal level for a comparison to determine whether the pixel issaturated.

Signal branch 38 has a sample and hold (SH) element. This can include asampling switch 44 and a holding capacitor 46. Similarly, reset branch40 includes a sampling switch 48 and a holding capacitor 50, andcomparator branch 42 includes a sampling switch 52 and a holdingcapacitor 64.

Signal branch 38 and reset branch 40 are connected to the input nodes56, 58, respectively, of a differential amplifier 60.

Differential amplifier 60 amplifies the difference between the resetvoltage R and the signal voltage S to produce a difference voltage(R-S). This difference voltage is used to set the brightness value ofthe pixel. Pixels with higher difference voltages (R-S) appear brighterin the resulting image.

Comparator branch 42 is connected to an input node 62 of a comparatordevice 64. The other input node 66 of comparator 64 is connected to ananalog voltage, V.adj, that is adjusted to set a minimum signal levelcorresponding to a signal voltage of a saturated pixel.

The output of comparator 64 is connected to an analog multiplexer (MUX)70. The MUX controls a normally open switch 74 at the output ofdifferential amplifier 60 and a normally open switch 76 at the output ofa voltage source 78 set at V.aa.

These readout and comparator circuits can be common to entire column 34of pixels.

Each photodetector 26 in each pixel 22 in the array converts lightenergy received in the form of photons into an electrical charge. Thatcharge corresponds to the amount of light that pixel receives prior to aread out function. Readout of pixel 22 for row 24 occurs in threestages: an image integration stage; a signal readout stage; and a resetstage.

Prior to readout, the voltage on photodiode 26 is reset voltage R. Resetvoltage R is typically about 3.0 V. After row 24 has been selected forreadout, this voltage is integrated and decreases in response toincident light.

During the integration stage, light energy in the form of photonsstrikes photodiode 26. Ideally each photon creates an electron-hole pairin the photodiode. Photodiode 26 is designed to limit recombinationbetween the newly formed electron-hole pairs. As a result, thephotogenerated holes and electrons migrate during the integration stage.The photogenerated holes are attracted to the ground terminal of thephotodiode, and the photogenerated electrons are attracted to thepositive terminal of the photodiode. Each additional electron attractedto the positive terminal of photodiode 22 reduces the voltage on thephotodiode from the reset value R. The integration stage typically takesabout 1 μsec.

During the signal readout stage, the resultant signal voltage S onphotodiode 26 is sampled onto signal branch 38 and comparator branch 42.In a fully integrated pixel, corresponding to a brightest pixel in theimage, the signal voltage S is about 1.5 volts.

During the reset stage, the value of the photodiode is reset to resetvoltage R by sampling V.aa onto photodiode 26 using reset transistor 28.This reset value is sampled onto the reset branch 42. The resetoperation typically takes about 1 μsec.

When column 34 is selected, the voltages S and R stored on holdingcapacitors 46 and 50, respectively, are transferred to differenceamplifier 60. The sensed light level read out from the pixel, that isthe number of photons incident on photodiode 26, is proportionate to thedifference between the reset level R and the signal level S determinedby difference amplifier 60.

When column 34 is selected, the signal voltage S stored on holdingcapacitor 54 is transferred to input node 62 of comparator 64 andcompared to V.adj. Comparator 64 outputs a LOW value for S>V.adj and aHIGH value for S<V.adj which occurs at a light level 104.

FIG. 3 illustrates the response of signal voltage S, reset voltage R,and difference voltage (R-S) to an incident light level. At asufficiently high incident light level, saturation light level 102,photodiode 26 becomes unable to absorb additional photons during theintegration period and saturates. When this happens, signal voltage Sdrops to a saturation voltage V.sat, typically about 1.5 V, and remainsconstant at V.sat for all light levels above saturation light level 102.

As shown in FIG. 3, V.adj is set slightly above V.sat to ensure thatcomparator 64 will output a HIGH level when signal voltage S equalsV.sat.

If signal voltage S is greater than V.adj, corresponding to anunsaturated pixel, comparator 64 outputs a LOW value and MUX 70 controlsswitch 74 at differential amplifier 60 to close, passing differencevoltage (R-S) to the rest of the signal processing chain.

Signal voltage S will fall below V.adj for all light levels below anadjusted saturation light level 104 which is slightly below saturationlevel 102. Consequently at light level 104 the pixel is nearlysaturated.

If signal voltage S is less than V.adj, corresponding to a (nearly)saturated pixel, comparator 64 outputs a HIGH value and MUX 70 controlsswitch 76 at voltage source 78 to close, passing V.aa to the rest of thesignal processing chain. As shown in FIG. 3, V.aa is greater thandifference voltage (R-S) for all light levels. Thus, the pixel is readas having a brightest value.

An APS according to the present embodiment prevents artifacts in theresultant image by replacing the value output from a saturated pixelwith a maximum value, V.aa, for all light levels above an adjustedsaturation light level 104 at which the pixel is nearly saturated.

Artifacts can be produced in conventional CMOS imagers because the resetvoltage R, while ideally constant, in fact drops gradually in responseto increasing light levels. This effect is due to pixel 22 still beingexposed to light and hence producing electron-hole pairs during thereset stage. Since there is a finite time between setting the photodiodeto the reset voltage R and sampling the reset voltage onto reset branch40, electrons generated in the photodiode 26 by incident photons duringthe reset stage can migrate and reduce the voltage on the positiveterminal of photodiode 26.

In Region I of FIG. 3, corresponding to the normal operating mode of thesensor, reset voltage R is relatively constant compared to S. However inRegion II, reset voltage R gradually drops while signal voltage Sremains constant at V.sat. Consequently, difference voltage (R-S)continues to drop with increasing light levels. In a conventional CMOSimager, the increasingly oversaturated pixel, which should appear tobrighten in the image, actually reads out as a darkening pixel.

As shown in Region III, at an extremely high light level 106, resetvoltage R will also saturate at V.sat and difference voltage (R-S) willequal zero, representing a black pixel. Regions of such oversaturatedpixels produce artifacts in conventional CMOS imagers. FIG. 1illustrates such an artifact 14 in an image 10 produced with aconventional CMOS imager in which the center of the sun 12 appearsblack.

Since the output of pixel 22 is fixed at a maximum value V.aa for alllight levels above that at which pixel 22 first saturates, the problemassociated with reset voltage R approaching V.sat at extremely highlight levels are eliminated. Consequently, no artifacts are produced inthe resulting image.

According to another embodiment, the APS CMOS device includes on-chipanalog-to-digital conversion (ADC) circuitry 110, as shown in FIG. 4.The ADC circuitry is contained in a digital block 112. Analog signalsread out from pixel 22 are converted to digital signals which areprocessed in the digital block 112.

The digital signals have a maximum value corresponding to the brightestlevel of the pixels in the image. For example, a maximum digital valuefor an 8-bit sensor would be 255 (corresponding to binary value“11111111”). Digital block 112 can be adapted to output the maximumdigital value for analog voltage V.aa output by the sensor at all lightlevels above adjusted saturation level 104.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the photodetector shown in FIG. 2 as photodiode 26 could be anyphoto-sensing element such as a photogate or pinned photodiode.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. An active pixel sensor comprising: at least onepixel comprising a photodetector that outputs an output level indicativeof incoming light; a storage element, electrically connected to thephotodetector, operating to store said output level during aphotodetector readout operation; a comparison element electricallyconnected to said storage element operating to compare said output leveland an adjusted saturation voltage; and an output selector electricallyconnected to said comparison element, operating to output an adjustedmaximum voltage during pixel readout operation in response to thecomparison element determining that said adjusted saturation voltageexceeds said output level.
 2. The active pixel sensor of claim 1 furthercomprising an adjusted saturation voltage source electrically connectedto said comparison element operating to provide said adjusted saturationvoltage.
 3. The active pixel sensor of claim 1 further comprising anadjusted maximum voltage source electrically connected to said outputselector operating to provide said adjusted maximum voltage.
 4. Theactive pixel sensor of claim 1 wherein the photodetector is selectedfrom the group comprising a photodiode, a photogate, and a pinnedphotodiode.
 5. The active pixel sensor of claim 1 wherein the adjustedsaturation voltage is greater than a voltage at which the photodetectorsaturates.
 6. The active pixel sensor of claim 1 wherein the comparisonelement is a comparator.
 7. The active pixel sensor of claim 1 whereinthe adjusted maximum voltage is an operating voltage of the at least onepixel.
 8. The active pixel sensor of claim 1 further comprising ananalog-to-digital converter electrically connected to the outputselector and operating to convert the adjusted maximum voltage to amaximum digital value.
 9. The active pixel sensor of claim 8 wherein themaximum digital value is an 8-bit word having the value “11111111”. 10.The active pixel sensor of claim 1 further comprising a differencingelement electrically connected to the storage element and operating tocalculate a difference voltage from said output level and a reset level.11. The active pixel sensor of claim 10 wherein the output selectoroperates to output said difference voltage during the pixel readoutoperation in response to the comparator determining that the outputlevel exceeds the adjusted saturation voltage.
 12. A method comprising:storing an output level indicative of incoming light output from aphotodetector during a photodetector readout operation; comparing saidoutput level to an adjusted saturation voltage; outputting an adjustedmaximum voltage during a pixel readout operation in response to saidadjusted saturation voltage exceeding said output level; storing a resetlevel from the photodetector during the photodetector readout operation;calculating a difference voltage from said output level and said resetlevel; and outputting said difference voltage during the pixel readoutoperation in response to said output level exceeding said adjustedsaturation voltage.
 13. The method of claim 12 wherein said adjustedsaturation voltage is greater than a voltage at which the photodetectorsaturates.
 14. The method of claim 12 further comprising converting saidadjusted maximum voltage to a maximum digital value.